1. Field of the Invention
The present invention relates to a purification process of aliphatic polyester which is useful as a biodegradable polymer for substituting medical materials and general purpose resins. More particularly, the invention relates to a purification process of aliphatic polyester in order to obtain aliphatic polyester which scarcely contains unreacted monomers and low molecular weight volatile components.
2. Description of the Related Art
In an aliphatic polyester, polylactic acid can be prepared from a cyclic dimer of lactic acid which is usually called lactide and the preparation process has been disclosed in U.S. Pat. No. 1,995,970, 2,362,511 and 2,683, 136.
A copolymer of lactic acid and other hydroxycarboxylic acids can be usually prepared from lactide which is a cyclic dimer of lactic acid and a cyclic ester intermediate of hydroxycarboxylic acid (usually glycolide which is a dimer of glycolic acid). The preparation process has been disclosed in U.S. Pat. 3,636,956 and 3,797,499. When the polyester is prepared by direct dehydration polycondensation of lactic acid or lactic acid and other hydroxycarboxylic acids, azeotropic dehydration condensation of raw materials, that is, lactic acid or lactic acid and other hydroxycarboxylic acids is carried out preferably in the presence of an organic solvent and catalyst. The azeotropically distilled solvent is dehydrated and substantially anhydrous solvent is returned to the reaction system. Such process can provide high molecular weight polylactic acid and a copolymer of lactic acid and other hydroxycarboxylic acids which have practical strength (EP Publication 0572675).
As to preparation processes of aliphatic polyester from aliphatic polyhydric alcohols and aliphatic polybasic acids, a process for providing high molecular weight aliphatic polyester having practical strength has been disclosed in Japanese Laid-Open Patent Publication HEI 4-189822 and 4-189823.
Japanese Laid-Open Patent Publication SHO 62-25121 has disclosed a process for preparing high molecular weight polyglycolide or polylactide by addition of a phosphoric acid or phosphorus acid compound in the course of polycondensation. The addition decreases activity of a tin catalyst and inhibits decomposition of the polymer which takes place with the polycondensation reaction in the preparation process of polyglycolide or polylactide by direct polycondensation of glycolic acid or lactic acid in the presence of the tin catalyst. However, the weight average molecular weight of the polymer thus obtained is a few ten thousand at the highest (about 10,000 as a number average molecular weight), and hence, polymers which can endure practical use cannot be obtained by the method.
In these processes, the polymer obtained by melt polymerization is pelletized as intact and thus the catalyst used remains in the polymer. The residual catalyst is liable to cause a depolymerization reaction of the polymer due to heating in the later melt processing step and leads to a problem of causing deterioration of properties by a decrease in the molecular weight of the polymer. Further, monomers used as the raw material in these processes inevitably remain in an amount of several percents as unreacted materials in the polymer. It has also been known that lower boiling point impurities formed by the side reaction in the course of polymerization and low molecular weight volatile substances such as linear and cyclic oligomers remain in the polymer. The unreacted monomers and low molecular weight volatile substances which remain in the polymer lead to the deterioration of storage stability and processability of the polymer or, lead to a decrease in the strength of formed materials.
Consequently, the residual catalyst must be removed by purification in order to obtain an aliphatic polyester which has a sufficiently high molecular weight and does not contain the catalyst. For example, Japanese Laid-Open Patent Publication SHO 63-145327 has disclosed a process for dissolving a catalyst containing polymer in an organic solvent which is immiscible with water, and removing the catalyst by bringing the resultant solution into contact with water or an aqueous layer containing an inorganic acid, water soluble organic acid or water soluble complexing agent. The process, however, decreases contact efficiency for the aqueous layer when the polymer solution becomes viscous, and thus has a problem of requiring treatment with a dilute solution of the polymer having a concentration of 0.5-4.0% by weight or a problem of poor separation after mixing the organic solvent solution with the aqueous layer. Japanese Laid-Open Patent Publication SHO 63-254128 has disclosed a purification process by adding a precipitating agent to the polymer solution in a field of turbulent shear flow.
However, the process has poor efficiency for catalyst removal because catalyst removal and crystallization of the polymer are simultaneously conducted. Problems have also been found on application of the process in industry because of the need for specific equipment. The process for removing the catalyst from polyester derived from aliphatic polyhydric alcohols and aliphatic polybasic acids has not yet been known.
As to a process for reducing the content of unreacted monomers and low molecular weight volatile substances, Japanese Laid-Open Patent Publication SHO 62-64824 has disclosed a purification process of the polymer by reprecipitation after polymerization. In the process, the polymer is dissolved in a good solvent such as chloroform and the solution obtained is poured into a lean solvent such as methanol to precipitate the insoluble polymer alone and to remove the soluble monomer. The process, however, is unfavorable in industry because the steps are complex and additionally the yield of the polymer decreases.
Japanese Laid-Open Patent Publication HEI 3-14829 has described a process for preparing a bioabsorbable polyester by reaction of glycolide and/or lactide. The process can prepare polyester having a low content of unreacted monomers and residual low molecular weight volatile substances by treating the reaction system under reduced pressure in the second half of the polymerization reaction and after the end of the reaction while maintaining the polymer in a molten state. However, the polymer prepared by the process contains 0.3-0.9% of unreacted monomers and the active catalyst remains in the polymer. Thus, the polymer is insufficient in heat resistance and weatherability and cannot provide a formed product which is durable for a long period of use.